1. Field of the Invention
The field of art to which this invention pertains is the solid bed adsorptive separation of isomeric dinitrotoluenes. More specifically, the invention relates to a process for separating 2,4-dinitrotoluene from the other dinitrotoluene isomers by employing a solid bed adsorption system.
2. Background Information
The isomers, 2,4-dinitrotoluene and 2,6-dinitrotoluene are important starting materials for making polyurethanes which are useful in many applications as rigid or flexible forms or as fibers, e.g., insulation, soundproofing, interlinings for clothing and sleeping bags, cushions, spandex, etc.
It is common industrial practice to make polyurethane from a mixture of the isomers, 2,4- and 2,6-toluene diisocyanate (TDI), for example 80/20 or 65/35, derived from 2,4- and 2,6-dinitrotoluene, because it is difficult and expensive to separate them by existing techniques. Current methods of separating the isomers involve crystallization, e.g. U.S. Pat. No. 4,246,187, and hence, are energy-intensive and time-consuming. Moreover, polyurethanes derived from pure 2,4-dinitrotoluene have dramatically different properties compared to materials synthesized from mixtures. It is further expected that polyurethanes synthesized from (relatively) pure 2,6-dinitrotoluene will have different and more useful properties, for example, a higher glass transition temperature, Tg, and greater stability. Also, the reaction speed with a single isomer is expected to be more consistent. Accordingly, it is desirable to separate the TDI isomers or their precursors, the dinitrotoluenes, by an economical process.
It is well known in the separation art that certain crystalline aluminosilicates can be used to separate hydrocarbon types from mixtures thereof. Furthermore, X and Y zeolites have been employed in a number of processes to separate individual hydrocarbon isomers.
It is known from U.S. Pat. No. 3,069,470 to Fleck et al, to use type X zeolites for the separation of the meta isomer from other isomers of toluidine. From U.S. Pat. No. 4,480,129, it is known that X and Y type zeolites, exchanged with transition metals, are paraselective in a mixture of isomers of toluidine. In Ser. No. 537,123 owned by the present assignee, it is disclosed to separate p-toluidine from its isomers with an X- or Y-type zeolite exchanged with Fe, Mn, Co, Ni or Zn and a desorbent comprising aniline or, when using an X-type zeolite, alkylamine having greater than eight carbons.
In Japanese Patent Application No. 56905/79, publicly disclosed on Nov. 20, 1980, it is disclosed that a solid adsorbent containing titanium oxide will selectively adsorb the p-isomer of toluidine.
It is known from U.S. Pat. No. 4,270,013 to Priegnitz et al that ortho-nitrotoluene may be separated from other nitrotoluene isomers by using a type X zeolite containing at exchangeable cationic sites one cation selected from a group that includes potassium and barium. The specific desorbent materials disclosed by this reference are toluene and 1-hexanol. The separation of isomers of disubstituted benzenes with crystalline aluminosilicates having silica/alumina mole ratio of at least 12 is disclosed in U.S. Pat. No. 4,467,126 to Zinnen.